What Penetrating Damp Actually Is
Penetrating damp is moisture entering a building from outside – through a defect in the roof, walls, render, pointing or rainwater goods – rather than rising from the ground or condensing from indoor air. It’s often more noticeable after heavy or prolonged rainfall, since that’s when the defect is under the most pressure.
Why The Damp Patch Isn’t Always Where The Water Gets In
This is the part that catches a lot of people out. Water doesn’t always travel straight through a wall in a direct line from entry point to internal surface.
“The damp patch may not necessarily be at the site of the problem.” — Cadw
Where moisture becomes trapped behind a less permeable material, it can travel sideways or downwards through the wall before emerging somewhere more permeable – which might be a foot or more from the actual defect. This is one of the reasons a proper investigation looks at the whole wall and roof, not just the spot where the damp is showing.
Common Causes
Roof, chimney and rainwater disposal defects are consistently identified as the most common sources of this kind of localised damp – which is why an inspection should always include the roof, not just the walls at ground level.
How It’s Diagnosed
A proper investigation checks the property from the outside in – the roof, flashing, chimney masonry, brickwork, pointing, render, window and door surrounds, and rainwater goods – rather than starting from the internal damp patch and assuming that marks the source.
It’s also worth ruling penetrating damp in or out before assuming a low-level patch is rising damp. A damp patch near the base of a wall can just as easily be water that’s travelled down through the wall from a defect higher up, as moisture rising from the ground.
The Bottom Line
Fixing penetrating damp means finding the actual defect letting water in – which isn’t always directly behind the patch you can see – and repairing that, rather than treating the symptom on the inside wall.
Frequently Asked Questions
If the damp patch is at the bottom of a wall, does that rule out penetrating damp?
No. Water can travel down through a wall from a defect higher up – a roof, flashing or brickwork problem – and emerge much lower down. A low-level patch doesn’t automatically mean rising damp.
Why does penetrating damp get worse after heavy rain?
Because the external defect letting water in – a crack, a gap in pointing, a damaged roof covering – is under more pressure during and after heavy or prolonged rainfall, allowing more water through than in dry weather.
Can penetrating damp happen through a cavity wall?
Yes. Debris left in a cavity during construction, or a damaged cavity tray, can allow moisture to bridge across to the inner leaf and appear as damp internally, even though the outer wall itself may look fine.
Is penetrating damp only a problem in older buildings?
No. While solid-walled older properties are often more exposed to the effects of a defect, penetrating damp can affect any property, of any age, wherever there’s a genuine defect in the roof, walls, render, pointing or rainwater goods.
Water Getting In Somewhere It Shouldn’t?
Get in touch and we’ll trace the source before recommending any repair – no pressure, just an honest assessment.